1. Field of the Invention
The present invention concerns a method to generate magnetic resonance image data of an examination subject with the use of a magnetic resonance imaging system, as well as a corresponding reconstruction device and a magnetic resonance imaging system.
2. Description of the Prior Art
Imaging systems that are based on detection of magnetic resonance signals produced by nuclear spins, known as magnetic resonance tomography systems, have been successfully established and proven in a multitude of applications. In this manner of image acquisition, a static basic magnetic field B0 that serves for the initial alignment and homogenization of magnetic dipoles in material to be examined, is superimposed with a rapidly switched magnetic field (known as a gradient field) for spatial resolution of the imaging signal. To determine material properties of an examination subject to be imaged, the dephasing or relaxation time after a deflection of the magnetization out of the initial alignment is determined, such that various relaxation mechanisms or relaxation times that are typical to the material can be identified. The deflection typically takes place with a number of RF pulses, and the spatial resolution is based on a temporally established manipulation of the deflected magnetization with the use of the gradient field, in a series of pulses known as a measurement sequence that establishes a precise chronological order of RF pulses, modification of the gradient field (by a switching sequence of gradient pulses), and the acquisition of measurement values.
If a switching sequence of the gradient field in a measurement sequence experiences a time deviation relative to an expected point in time of the switching (which is designated in the following as a “switching dilatation”), this leads to inaccuracies in the spatial resolution of the magnetic resonance signal that cause distortions and other errors in the magnetic resonance imaging of an examination subject.
An association between measured magnetization—from which the aforementioned material properties can be derived—and a spatial coordinate of the measured magnetization typically takes place with the implementation of an intermediate step. In this intermediate step, acquired raw magnetic resonance data are entered into a storage format known as “k-space”, wherein the coordinates of k-space are coded as a function of the gradient field. The gradient field modifies the resonance frequency (Larmor frequency) and, for example, also the phase position of the magnetization deflected by an RF pulse in a spatially dependent manner, such that a spatial information is obtained via identification of phase position and resonance frequency of the measured magnetization. In other words, spatial information is based on the coordinate system of k-space (spatial frequency) with phase and frequency coding, and is determined as a function of the gradient field. The magnitude of the magnetization (in particular of the transverse magnetization in a plane defined transverse to the previously described basic magnetic field) at a defined location of the examination subject can be determined from the readout point with a Fourier transformation that calculates the signal strength of the signal in the spatial domain from a signal strength (magnitude of the magnetization) that is associated with a specific frequency (the spatial frequency).
K-space thus forms an inverse Fourier space relative to the spatial domain of the examination subject, such that the magnetic resonance signals are transformed into the spatial domain through a Fourier transformation, in order to create the magnetic resonance image. The gradient field thus defines a point in k-space, and the curve of the change of the gradient field establishes a series of k-space points that can be designated as a “trajectory” through k-space, or as a “projection”.
In a disadvantageous case, the aforementioned switching dilatation in current magnetic resonance imaging systems can reach an order of microseconds, and therefore markedly exceed the switching delay of an RF pulse to deflect the magnetization. If this is the case, the gradient field assumes a different value than the expected one at a readout point in time of the raw magnetic resonance data, and a gradient field or a phase position of the spins that corresponds to an expected k-space coordinate is achieved only at a later point in time. This results in the measured magnetic resonance signal being associated with a displaced coordinate in k-space, since the gradient field or the required phase position of the spins at the measurement point in time does not have the expected value.
If a shift of the k-space coordinates of the trajectory takes place due to the switching dilatation so that an approximately coherent shift (explained in more detail later) is present for all trajectories (for example given line-by-line Cartesian sampling of k-space), the switching dilatation has nearly no effect on the quality of the imaging of the examination subject since the additionally arising phase is the same for all k-space points. However, if this is not the case—such as if the sampling of k-space is selected in a particular path through k-space and takes place radially, for example—this inevitably leads to severe artifacts in the imaging. In this case, a correction of the k-space points of the trajectories should take place in order to be able to implement a transformation of the magnetic resonance signals in the positional space of the examination subject while avoiding distortions and image artifacts.
For example, for this a method for correction is known that corrects the k-space points of the trajectories with the aid of a correction value in order to thus avoid image artifacts. The correction value is added by entering the correction method as an input, wherein the input can take place manually or from a database, for example. However, this method is time-consuming or based on general models for the shift due to the switching dilatation, such that significant cost arises in the acquisition of the magnetic resonance imaging and, moreover, the correction does not always occur optimally.